The present invention relates to the technical field of taking/recording radiation image using a focusing grid, in particular, to a radiation imaging apparatus that enables an appropriate radiation image to be obtained even if a radiation source is moved.
In breast cancer screening, the success rate of early detection that is achieved through physical examination by inspection and palpation only is improved if it is combined with mammography by a radiation image taking apparatus solely intended for breasts (which is hereinafter referred to as a breast imaging apparatus); hence, breast cancer screening that is currently practiced involves the use of a breast imaging apparatus in addition to (or in place of) inspection and palpation.
The breast imaging apparatus comprises an imaging table enclosing a radiation image recording medium (which is hereinafter referred to as an imaging medium); to operate it, the breast is placed on the imaging table, compressed with a press plate, irradiated with a radiation from the press plate side; the radiation transmitted through the breast is received by the imaging medium and its radiation image is taken on the imaging medium.
The breast is composed of mammary gland tissue, fatty tissue, skin, etc. and in order to take a picture of the breast required for correct diagnosis, it is necessary that the breast be compressed to the highest possible uniformity in thickness to enable the imaging of the mammary gland and the fatty tissue in detail. In addition, in order to record a sharp image, it is preferred that the source of radiation be positioned right above the compressed breast so that an incoming radiation is normal to the breast.
A problem with the taking of radiation image is that scattered rays (scattered X-rays) will inevitably occur due, typically, to the tissue of an object such as the breast. If such scattered rays get into the detector, deterioration of image quality such as lower contrast will occur.
Under the circumstances, a radiation image taking apparatus such as a breast imaging apparatus is usually provided with a grid (a scattered rays removing grid) just in front of the detector (or its radiation-receiving plane) in order to block the entrance of scattered rays into the detector so that they will not cause deterioration of image quality (see, in particular, JP 2004-177251 A).
In one example, the grid consists of a lead plate or other radiation-opaque material that alternates with a similar plate form of a radiation-transparent material in a direction perpendicular to the length of each plate; in the case of a breast imaging apparatus, the two materials are arranged such that they alternate in the transverse direction of the subject (parallel to her chest wall).
The grid is also known to be available in two types, one being a parallel grid in which the radiation-transparent areas are formed parallel to each other and the other being a focusing grid (refer to FIG. 6) in which the radiation-transparent areas diverge progressively in the direction of radiation transmission so that the focus of convergence will coincide with the radiation source (the focus of X-rays).